xlviii 



INTRODUCTION. 



TABLE 61. D2iraiio7i of sunshine at different latitudes for different values 

 of the suti^s declination. 



IvCt Z be the zenith, and NH the hori- 

 zon of a place in the northern 

 hemisphere. 



P the pole; 



QEO' the celestial equator; 



RR' the parallel described by the sun on 

 any given day; 



S the position of the sun when its upper 

 limit appears on the horizon; 



PN the latitude of the place, <^. 



ST\h& sun's declination, S. 



PS the sun's polar distance, 90° — S. 



ZS the sun's zenith distance, z. 

 ZPS the hour angle of the sun from meridian, /. 



r the mean horizontal refraction = 34' approximately. 

 s the mean solar semi-diameter =16' 



^ = 90° + r + 5 = 90° 50' 



In the spherical triangle ZPS, the hour angle ZPS may be computed 

 from the values of the three known side by the formula 



sm 



or 



\ZPS= I sin i {ZS + PZ-PS) sin j {ZS + PS-PZ) 

 N sm PZ sin PS 



sini/= I sin i (^ + 8 — <^) sin i (^ - 8+ <^) 



cos ^ cos 8 



The hour angle t, converted into mean solar time and multiplied by 2, 

 is the duration of sunshine. 



Table 61 has been computed for this volume by Prof Wm. Libbey, jr. 

 It is a table of double entry with arguments 8 and ^. For north latitudes 

 northerly declination is considered positive and southerly declination as 

 negative. The table may be used for south latitudes by considering 

 southerly declination as positive and northerly declination as negative. 



The top argument is the latitude, given for every 5° from 0° to 40°, for 

 every 2° from 40° to 60°, and for every degree from 60° to 8or 



The side argument is the sun's declination for every 20' from S 23° 27' 

 to A^23°27'. 



The duration of sunshine is given in hours and minutes. 



To find the duration of sunshine for a given day at a place whose 

 latitude is known, find the declination of the sun at mean noon for that day 

 in the Nautical A lma7iac, and enter the table with the latitude and declination 

 as arguments. 



